In vehicles (referred to as a new transport system, AGT (Automated Guideway Transit), or APM (Automated People Mover)) for track-based transportation system traveling on a track while being guided by a guide rail provided in the track, other passenger vehicles for railway, or vehicles such as a bus, a seat may be provided, which is installed inward in a width direction of the vehicle inside a side wall provided on a side in a travel direction (forward and backward directions) of the vehicle. PTLs 1 and 2 disclose a seat in which a seatback is fixed to a vehicle inner side surface of a side wall.